Oceans in Motion

• The ocean is constantly in motion

• Driven by:

–Winds

–Currents

–Coriolis Effect

• WINDS ARE NAMED WINDS ARE NAMED FROM WHICH THEY FROM WHICH THEY BLOWBLOW

• CURRENTS ARE CURRENTS ARE NAMED BY THE NAMED BY THE DIRECTION THEY FLOWDIRECTION THEY FLOW

What causes ocean currents?

• Winds produce a stress on the ocean and cause it to move– Wind-Driven Circulation

• Density differences created by the exchange of heat and moisture with the atmosphere cause movement– Thermohaline Circulation

How does the Ocean Circulate?

• Similar to a two story building…

• There is an upstairs and downstairs

• What goes on in each floor is separated by the floor or ceiling

• Connections between the two floors occurs only at limited locations

• Where elevators or stairways are found

Mixing

• In the ocean’s two stories, there is the surface and deep ocean.

• The floor that separates them is called the pycnocline– Effectively restricts direct mixing across this

level

• Connections between the two parts of the ocean are only in limited regions

Cyclones and Hurricanes

• Doldrums – convergence of trade winds fuel tropical cyclones and hurricanes

OCEAN CIRCULATION

• Ocean water circulates in currents

• Surface currents are caused by wind

= 10% of water movement

• The other 90% is driven by gravity

dense water sinks, less dense rises

Surface Mix Layer

Deep Water Layer

4000m

100-200m

100-200m

0m

Upwelling Downwelling

wind

sun

light

Density

driven

When several containers of children's bathtub toys spilled over a ship's side and were released into the Pacific Ocean, who would've thought a concerted research project to study the ocean's currents would be the result?

January 10, 1992….

• In January 1992, a container ship near the International Date Line, headed to Tacoma, Washington from Hong Kong, lost 12 containers during severe storm conditions.

• One of these containers held a shipment of 29,000 bathtub toys.

• Ten months later, the first of these plastic toys began to wash up onto the coast of Alaska.

• Driven by the wind and ocean currents, these toys continue to wash ashore during the next several years and some even drifted into the Atlantic Ocean.

60,000 Nike shoes wash ashore from

ship spill

Scientists use these spills to help study ocean circulation

• This could help with oil spills and predicting in which direction the oil will travel

• Learn about pathway of currents and how they are changing and what affects them.

A Bad Break in Pool

How do the winds make the ocean move?

Not the worst one ever, just a bad one

A Bad Break in Pool

• cue hits at bad angle• cue moving too slowly• billiard balls poorly racked up

A “Bad Break” in the OceanAir molecules hit water molecules at bad angle, AND:• H2O’s irregularly spaced• hydrogen bonded together• imagine golf ball hitting billiard balls that are poorly racked

N2O2

Transfer of Kinetic Energy from Air to Ocean

N2O2

• BILLIONS of air molecules hit the ocean every second• The upper ~100 m of the ocean moves in response to windsBUT:the water does not move in EXACTLY the same direction as wind

Sverdrup et al., Introduction to the World’s Oceans, 8th edition, McGraw Hill, Fig. 9.1

Ekman Transport • Wind bombards the surface ocean• Water starts to move• Coriolis deflection alters its path

What happens UNDER the surface?

• Remember that the Remember that the Earth is rotating Earth is rotating clockwise – to the clockwise – to the east. east.

• Objects traveling in Objects traveling in Northern Northern hemisphere are hemisphere are deflected to right deflected to right (clockwise)(clockwise)

• Southern Southern hemisphere are hemisphere are deflected to left deflected to left (counterclockwise)(counterclockwise)

The Coriolis EffectThe Coriolis Effect

1.1. The Earth’s rotation affects linear The Earth’s rotation affects linear movement – it is deflected, the path is movement – it is deflected, the path is not linear, rather it is curvilinearnot linear, rather it is curvilinear

2. The effect is too subtle to notice over 2. The effect is too subtle to notice over short distances, but things which travel short distances, but things which travel over long distances such as airplanes, over long distances such as airplanes, winds, and water currents display the winds, and water currents display the Coriolis effect.Coriolis effect.

Ekman “Spiral”

• surface layer (1) drags onthe water underneath (2). 1

3

2

• layer 2 starts to move.It moves more slowlythan layer 1 due tofriction.(smaller yellow arrow)

• Coriolis deflectionalters path of layer 2.

Ekman “Spiral”

• layer 2 drags on the water underneath(layer 3).

1

3

2

• layer 3 moves more slowly

• Coriolis deflection alters path of layer 3.

Ekman “Spiral”

1

3

2

• The result? •A spiral pattern.

• AVERAGE MOTIONof the upper ~100mof the ocean is 90˚to the wind.

Building the currentsSTEP 1:Ekman Transport

• Winds drive E-W currents

• Zones of convergenceand divergence

• Continents get in the wayEXCEPT around Antarctica(stormiest ocean on Earth)

• Pattern of circulatingGYRES.

Sverdrup et al., Introduction to the World’s Oceans, 8th edition, McGraw Hill, Fig. 9.4

Geostrophic FlowSargasso Sea - a mound of water in the Atlantic Ocean

Coriolis deflection is piling up water.

Gravity is pulling it down.

The pile is ALWAYS THERE. These forces must be EQUAL.

Sargasso Sea “Sea of Weeds”

Sargasso Sea Home to Many…

Gyres• Circuit of mid-latitude currents

around the perimeter of an ocean basin.

• Example: the Gulf Stream, the North Atlantic Current, the Canary Current, & the North Equatorial Current make up the North Atlantic Gyre

Five major gyres and the Antarctic circumpolar current

Gyre Formation

Warm

Cool

CLOCKWISE

COUNTER CLOCKWISE

North Atlantic Gyre

•Notice how the winds are

helping the currents.

•This also shows the effects of the Eikman spiral

What happens when you push in on a plastic container of water?

The container moves first, and water moves a

fraction of a second later.

Water sloshes up again the left-hand side, creating a pile

Sverdrup et al., Introduction to the World’s Oceans, 8th edition, McGraw Hill, Fig. xxx

Earth’s rotation

As Earth rotates the continents smack into the oceans

Asia, Australia hit thePacific Ocean

The Americas hit theAtlantic Ocean

Continents crash into the mound of water.The mound is asymmetrical.This leads to Western Intensification – water piles up on the coastWest East

NorthAmerica

A West to East Cross Section of the mound of water

1m

Only ~ 1m high, but that’s enough to create BIGdifferences in the currents.

Western Intensification

West East

NorthAmerica

A West to East Cross Section of the mound of water

Gravity is trying to pull this down and out to flatten the ocean.

The flow is constricted on the WEST side and spread out on the EAST

Think about constricting flow out of a garden hose by covering halfthe opening with your thumb. The constricted flow moves FASTER.

Western Intensification

West East

NorthAmerica

Think about constricting flow out of a garden hose by covering halfthe opening with your thumb. The constricted flow moves FASTER.WHY?

SAME AMOUNT of water forced to move through smaller opening

Canary Current(Flowing OUT of screen)diffuse, slow

Gulf Stream(Flowing into screen)Narrow and fast

WesternWestern vs. vs. Eastern CurrentsEastern Currents

Western CurrentsWestern Currents Eastern CurrentsEastern Currents• Gulf Stream, Kuroshio,Gulf Stream, Kuroshio, East Australia CurrentsEast Australia Currents

• warm water moving fromwarm water moving from equator to poleequator to pole

• narrow: < 100 km widenarrow: < 100 km wide

• deep: down to 2 kmdeep: down to 2 km

• ““fast:” 100s of km/dayfast:” 100s of km/day

• sharp boundaries defined bysharp boundaries defined by water temperaturewater temperature

• California, Canary, PeruCalifornia, Canary, Peru CurrentsCurrents

• cold water moving fromcold water moving from pole to equatorpole to equator

• wide: ~ 1000 km widewide: ~ 1000 km wide

• shallow: < 500 mshallow: < 500 m

• ““slow:” 10s of km/dayslow:” 10s of km/day

• diffuse boundariesdiffuse boundaries

Sverdrup et al., Introduction to the World’s Oceans, 8th edition, McGraw Hill, Fig. 9.14

Coupled Surface and Deep water Circulation

Convergence:water masses collideand sink downwelling

Divergence:Surface waters move APART.Deep water rises up to fillthe gap upwelling

Back to Upwelling &Downwelling

Upwelling brings NUTRIENTS(from DECAYED organicmatter) back up to the surface.

Sverdrup et al., Introduction to the World’s Oceans, 8th edition, McGraw Hill, Fig. 9.3

• Area where trade winds converge

• Meteorlogical equator = Thermal equator

• Moves north & south of the Earth’s equator

ITCZ =Inter-Tropical Convergence Zone

Sverdrup et al., Introduction to the World’s Oceans, 8th edition, McGraw Hill, Fig. 9.11

Major Zones of Convergence and Divergence